A better way for customizing parts of gnus

[victor@algebraic.org (Victor S. Miller)]

At work we have emacs 21.2 installed (solaris), but I noticed that
some parts of gnus are not working quite right.  After a bit of
investigation I found that in site-lisp there are a number of el (and
elc) files from gnus (which override the standard ones in the
distribution) which are dated 1994!  Further investigation shows that
they are there for various local modifications.  For instance
message.el is modified in order to add some mandatory message headers,
whose contents are checked for validity.  To do this a new defvar and
some new defun's are added.  I think that the way in which they've
done it is terrible, but the question is -- what would be the best way
to do it -- for example have some hook defined to load files which
will contain the supplementary definitions.  If so, which hook should
it be?  For various reasons it is not desirable to have all users
modify their .emacs or .gnus files.  How can one get these hooks
initialized so that everybody always gets them?

Victor